This invention relates to cross-linked cellulose fibers and, more particularly, to cellulose pulp fibers which have been cross-linked with formaldehyde.
It is known that new characteristics, especially a greatly increased water absorptivity, can be imparted to cellulose fibers and particularly to wood fibers or cotton linters by subjecting the fibers to a cross-linking reaction. The cross-linking of the cellulose fibers is a double etherification of the primary alcohol groups of the anhydrogluclose units with the cross-linking agent. Cross-linking agents which have been suggested for use include formaldehyde, polyoxymethylene, trioxane, aminoplasts, and glyoxal in the presence of a low molecular weight organic acid, i.e., Lewis acids, as the catalyst. Other cross-linking components such as epichlorohydrin or other epoxides are reacted with cellulose in the presence of a basic catalytst. It is accepted in the art that the links between the anhydroglucose units of the cellulose chains hinder the formation of inter-fiber hydrogen or hydrate bonds, imparting stiffness to the fibers and increasing the water absorption through capillarity.
Cross-linked cellulose fibers have been employed in the preparation of napkins, sanitary pads, and diapers; and also in the preparation of sheet materials having improved bulk, softness, as well as reduced tensile strength. If employed with a resin binder, the modified fibers are particularly useful in the manufacture of non-wovens characterized by their improved softness, bulk, caliper, and absorbency.
Cross-linked cellulose fibers are disclosed in, for example, French Pat. No. 892,799 and U.S. Pat. No. 2,010,635. However, difficulties associated with the uniform cross-linking of cellulose fibers without destruction of other properties has led to many attempted improvements in the processes employed in providing cross-linked fibers as seen from U.S. Pat. No. 3,224,926; U.S. Pat. No. 3,440,135, and U.S. Pat. No. 3,700,549. U.S. Pat. Nos. 3,224,926 and 3,440,135 describe processes which require an impregnation step with the cross-linking agent or with the catalyst and a drying or storage step for periods of time up to forty-eight hours, followed by a defiberizing step and a thermal treatment step. The aforesaid references suggest that formaldehyde, which is the least expensive cross-linking agent known and fully effective at low levels, is less desirable than other cross-linking agents because of its volatility. The use of more expensive cross-linking agents and/or the long-aging or drying times have prevented wide-scale commercial manufacture of the cross-linked fibers. Moreover, due to the non-uniformity of the heretofore known processes and also the adverse effect on the cross-linked fibers as a result of long thermal treatments and long contact with the reagents including acids, the products obtained have not been fully acceptable.
It is also known in the prior art that paper or cardboard can be treated with formaldehyde. Note U.S. Pat. Nos. 1,816,973; 3,264,054, and 3,310,363. The processes disclosed in the noted patents are attempts to improve the physical properties, especially wet tensile strength of the sheets, and are not directed to the treatment of individual cellulose fibers.
It is also recognized in the prior art that in the cross-linking of the cellulose fibers as set forth in French Pat. No. 2,224,485 there is an inter-relationship between the concentration of the fibers and reagents, and specifically the percentage of water employed. It has been suggested that the amount of water can be reduced by utilizing a solvent such as acetone. However, the use of large amounts of acetone and the need for the generation of the solvent catalyst mixture adversely influenced such methods from commercial use.